Human tuberculosis caused by Mycobacterium tuberculosis (M. tuberculosis) is a severe global health problem, responsible for approx. 3 million deaths annually, according to the WHO. The worldwide incidence of new tuberculosis (TB) cases had been falling during the 1960s and 1970s but during recent years this trend has markedly changed in part due to the advent of AIDS and the appearance of multidrug resistant strains of M. tuberculosis. 
The only vaccine presently available for clinical use is BCG, a vaccine whose efficacy remains a matter of controversy. BCG generally induces a high level of acquired resistance in animal models of TB, but several human trials in developing countries have failed to demonstrate significant protection. Notably, BCG is not approved by the FDA for use in the United States because BCG vaccination impairs the specificity of the Tuberculin skin test for diagnosis of TB infection.
This makes the development of a new and improved vaccine against TB an urgent matter, which has been given a very high priority by the WHO. Many attempts to define protective mycobacterial substances have been made, and different investigators have reported increased resistance after experimental vaccination. However, the demonstration of a specific long-term protective immune response with the potency of BCG has not yet been achieved.
Immunity to M. tuberculosis is characterized by some basic features; specifically sensitized T lymphocytes mediates protection, and the most important mediator molecule seems to be interferon gamma (IFN-γ).
M. tuberculosis holds, as well as secretes, several proteins of potential relevance for the generation of a new TB vaccine. For a number of years, a major effort has been put into the identification of new protective antigens for the development of a novel vaccine against TB. The search for candidate molecules has primarily focused on proteins released from dividing bacteria. Despite the characterization of a large number of such proteins only a few of these have been demonstrated to induce a protective immune response as subunit vaccines in animal models, most notably ESAT-6 and Ag85B, also called MPT59 (Brandt et al 2000).
Animal tuberculosis is caused by Mycobacterium bovis, which is closely related to M. tuberculosis and within the tuberculosis complex. M bovis is an important pathogen that can infect a range of hosts, including cattle and humans. Tuberculosis in cattle is a major cause of economic loss and represents a significant cause of zoonotic infection. A number of strategies have been employed against bovine TB, but the approach has generally been based on government organized programs by which animals deemed positive to defined screening test are slaughtered.
Alternative strategies in TB vaccine development such as subunit vaccines (Andersen, P. 1994, Horwitz et al 1995, Roberts, A. D. et al 1995), genetic immunization (Huygen et al 1996, Tascon et al 1996) and attenuated strains of M. tuberculosis (Guleria et al 1996) are currently being explored in many laboratories. Due to the complexity of the host immune response against tuberculosis and the genetic restriction imposed by major histocompatibility complex molecules (MHC), it has become clear that an effective subunit vaccine containing multiple epitopes may be required to ensure a broad coverage of a genetically heterogeneous population. The present inventors and others have previously demonstrated that vaccines based on a mixture of culture filtrate antigens can induce levels of protection similar to BCG in mice (Andersen, P. 1994, Horwitz et al 1995, Roberts, A. D. et al 1995), but so far only a few experimental vaccines based on a single antigen have proved successful in animal models (Brandt at al 2000, Huygen et al 1996, Tascon et al 1996).